Timed, pulsed fuel injection systems are required for diesel engines and stratified charge engines and are very useful in conventional spark engines, particularly when air pollution control requires very accurate metering of fuel. However, pulsed fuel injection systems are difficult to design without metering errors due to wave effects in the system which result in dynamically caused errors (particularly for high pressure systems where the bulk modulus compressibility of the fuel becomes important and for very high speed systems). Systems which meter accurately cylinder-to-cylinder and cycle-to-cycle often lose calibration with time, and tend to be prohibitively expensive. Moreover, pulsed fuel injection systems tend to produce bad atomization at the very low speeds of engine startup: this problem has contributed to the cold startup problems with diesel engines which have persisted to some degree despite very extensive engineering effort.
It is a purpose of the present invention to produce a pulsed injection system which is simply connectable to air-fuel control logic, which produces extremely accurate metering from inexpensive components not built to high tolerances, and which continues to give good metering at all speeds even as the flow characteristics of the working parts vary with wear and deposits for the life of the engine. It is a further purpose to produce an injection system which is adaptable to very high pressure in-cylinder injection as well as to lower pressure applications.
It is another object of the present invention to produce an injection system which is totally insensitive to dynamic effects in the various injection lines and which produces excellent metering at any practical engine speed (at least up to approximately 15,000 RPM).
It is yet another object of the present invention to produce an injection system which is almost totally free of metering errors due to variations in injection lines and injection nozzles with varying flow pressure characteristics.
It is also an object of the present invention to produce an injection system which produces high instantaneous flow rates and good atmoization even during engine startup, and which provides a sharp cutoff of flow to prevent nozzle dripping.
It is another object of the invention to produce an injection pump where fuel flow is a linear function of control valve position, to simplify fuel-air ratio control logic design.
These and other objects are attained by producing a system where each working part has a geometrically simple shape and a mathematically well defined and simple function (in both the static and dynamic sense), by minimizing the complexity of each working part's function, by eliminating sources of interaction between the parts which produce second order metering errors, and by assuring that the distances of fluid pressure wave travel between the metering parts of the system are so small that the desired fluid equilibrium conditions between the operating parts are achieved during the time available even at the highest system operating speed.
Specifically, the pump has a pressurizing but not a metering function; total flow metering is done simply and continuously by a variable metering orifice across a constant pressure drop; and this total flow is divided into equal volumes and delivered in pulses by the interaction of three accumulator chambers interconnected in a pressure cascading sequence by a rotary distributor to the injection nozzles. Each accumulator, when it is opened to the downstream accumulator, discharges its volume as a sort of fluid capacitor, so that flow delivery even at very low engine speeds is in the form of a high speed pulse sufficient to assure good atomization. Because the accumulators are close together, accumulator charging and discharging is always complete at rotary distributor closing and the disturbing dynamic pressure effects produced in the injection lines and nozzles have no effect whatever on the metering events. Each of the working parts of the system is of simple construction and most of the parts have wide structural tolerances, so that the sum cost of the system is much less than the cost of old injection pumps, which have traditionally been very expensive devices.
If injection pulse shaping is required, the addition of adjustable throttling valves between the n accumulator chambers 3i and the nozzles produces excellent pulse shaping. Also, the continuous fuel metering device can be readily adapted with a pressure maximum to assure that fuel volume per injection pulse cannot exceed a certain set value.
Metered fuel flows into an accumulator chamber 1 which accumulates volume only above a certain pressure p.sub.1 which is in intermittant contact through a rotary distributor with an accumulator 2 once between each injection pulse. Chamber 2 accumulates volume only above a pressure p.sub.2 &lt;p.sub.1, so that all fuel metered while accumulator 2 is in contact with accumulator chamber 1, as well as all volume stored in chamber 1 since the previous contact period, flows into chamber 2 during the 1-2 contact period. When chamber 2 is cut off from chamber 1, it is in intermittant sequential contact through a rotary distributor with one of the n injection nozzle lines, each of which has an accumulator chamber 3i which receives the volume accumulated in chamber 2 and stores it at a pressure p.sub.3i less than p.sub.2. The transferred volume in the chamber 3i is discharged through the ith injection nozzle. Accumulator chambers 1, 2, and 3i are constructed so that chamber volumes are invariant with pressures below pressures p.sub.1, p.sub. 2, and p.sub.3i. Therefore, variations in blowdown pressures between chamber 2 and the chambers 3i do not result in errors of the metering event, which occurs in the redundant interaction between chambers 1 and 2 and the continuous fuel flow metering system. The pressure cascading system has the advantage that so long as each injection nozzle i has an adequate flow rate at a pressure below p.sub.3i, the inequalities p.sub.1 &gt;p.sub.2 &gt;p.sub.3i hold, and the rotary distributor seals, the system produces excellent time series and cross section metering statistics over the full rage of operating speeds and over a wide range of the design parameters P.sub.a1, P.sub.a2, P.sub.a3i, etc. Therefore, injection nozzles and injection line lengths need not be matched, and the system can operate at ultrahigh speeds.
The system can be modified to produce the desirable square wave pulse shape with a maximum fuel flow per degree crank angle.
The system is also adapted with pressure constraint means to assure that injected volume per pulse cannot exceed a set maximum to preclude overrichening.